Free Astronomy Magazine March-April 2018

30 MARCH-APRIL 2018 SPACE CHRONICLES ern constellation of Vela (The Sails), has now been studied using the MUSE instrument on ESO’s Very Large Telescope in Chile. An interna- tional team of astronomers has found that one of the stars in NGC 3201 is behaving very oddly — it is being flung backwards and for- wards at speeds of several hundred thousand kilometres per hour, with the pattern repeating every 167 days. Lead author Benjamin Giesers (Georg-August-Universität Göttin- gen, Germany) was intrigued by the star’s behaviour: “It was orbiting something that was completely in- visible, which had a mass more than four times the Sun — this could only be a black hole! The first one found in a globular cluster by di- rectly observing its gravitational pull.” The relationship between Odd behaviour of star reveals lonely black hole hiding in giant star cluster by ESO G lobular star clusters are huge spheres of tens of thousands of stars that orbit most galax- ies. They are among the oldest known stellar systems in the Uni- verse and date back to near the be- ginning of galaxy growth and evolution. More than 150 are cur- rently known to belong to the Milky Way. One particular cluster, called NGC 3201 and situated in the south- B ackground and video: astronomers using ESO’s MUSE instru- ment on the Very Large Telescope in Chile have discovered a star in the cluster NGC 3201 that is behaving very strangely. It ap- pears to be orbiting an invisible black hole with about four times the mass of the Sun — the first such inactive stellar-mass black hole found in a globular cluster. This important discovery impacts on our understanding of the formation of these star clusters, black holes, and the origins of gravitational wave events. This artist’s impression shows how the star and its massive but invisible black hole companion may look, as they orbit each other in the rich heart of the globular star cluster. [ESO/L. Calçada /spaceengine.org ]